Mechanism of Proton Transport in Ionic-Liquid-Doped

Oct 30, 2013 - Milan Kumar and Arun Venkatnathan*. Department of Chemistry, Indian Institute of Science Education and Research, Pune 411008, India...
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Mechanism of Proton Transport in Ionic-Liquid-Doped Perfluorosulfonic Acid Membranes Milan Kumar and Arun Venkatnathan* Department of Chemistry, Indian Institute of Science Education and Research, Pune 411008, India ABSTRACT: Ionic-liquid-doped perfluorosulfonic acid membranes (PFSA) are promising electrolytes for intermediate/high-temperature fuel cell applications. In the present study, we examine proton-transport pathways in a triethylammonium-triflate (TEATF) ionic liquid (IL)-doped Nafion membrane using quantum chemistry calculations. The IL-doped membrane matrix contains triflic acid (TFA), triflate anions (TFA−), triethylamine (TEA), and triethylammonium cations (TEAH+). Results show that proton abstraction from the sulfonic acid end groups in the membrane by TFA− facilitates TEAH+ interaction with the side-chains. In the IL-doped PFSA membrane matrix, proton transfer from TFA to TEA and TFA to TFA− occurs. However, proton transfer from a tertiary amine cation (TEAH+) to a tertiary amine (TEA) does not occur without an interaction with an anion (TFA−). An anion interaction with the amine increases its basicity, and as a consequence, it takes a proton from a cation either instantly (if the cation is freely moving) or with a small activation energy barrier of 2.62 kcal/mol (if the cation is interacting with another anion). The quantum chemistry calculations predict that anions are responsible for proton-exchange between cations and neutral molecules of a tertiary amine. Results from this study can assist the experimental choice of IL to provide enhanced proton conduction in PFSA membrane environments.

1. INTRODUCTION Proton-exchange membrane (PEM) fuel cells have attracted enormous interest due to their ability to generate high energy density with low environmental impact. These fuel cells have also been investigated for a wide range of applications like electronic devices and vehicles.1−3 In these cells, fuels, such as hydrogen gas and methanol, are introduced at the anode and oxidize at the platinum catalyst to generate electrons and protons. Electrons move via an external circuit, whereas hydrated protons travel through the PEM. At the cathode, electrons, protons, and oxygen gas react to produce water, which is the only byproduct. The PEM facilitates proton transport and hence plays a key role in determining the efficiency of this fuel cell. Experimental and theoretical studies have primarily focused on the characterization of perfluorosulfonic acid (PFSA)-based PEM membranes,4−14 like Nafion, 3M, and short side-chain, due to high proton conductivity and stability. These membranes have a hydrophobic polytetrafluoroethylene backbone and hydrophilic side-chains terminated by a sulfonic acid group. The sulfonic acid group can release a proton to the neighboring water molecules even at low hydration.7 An increase in hydration shows a nanophase segregation of hydrophobic and hydrophilic domains, enabling water connectivity and enhanced proton conductivity.5,8−10 Because sufficient hydration of membranes is required to achieve desirable power density (current density and cell voltage) from PEM fuel cells, these devices are best operated at ∼90 °C. Further, higher temperatures (>100 °C) lead to dehydration of © 2013 American Chemical Society

the membrane, which affects its thermal, mechanical, and electrochemical stability.15−18 However, operation of fuel cells at intermediate/high temperatures can provide many advantages such as (a) nonhumidified inlet gases, (b) easier water management at the cathode, (c) higher performance of the Pt catalyst on electrodes with increased tolerance to carbon monoxide poisoning, and (d) utilization of generated heat. However, these advantages can be realized only by an appropriate choice of a solvent and/or a PEM, which can facilitate proton conduction at temperatures above 100 °C. One of the standard approaches is to replace water with phosphoric acid due to its higher boiling point (158 °C). Neat liquid phosphoric acid possesses the highest intrinsic proton conductivity, where proton transport occurs via an interplay of extended and polarized hydrogen bonds in a frustrated hydrogen-bonding network (Grotthuss mechanism19).20 Phosphoric-acid-doped polybenzimidazole membranes have been investigated as alternative electrolytes for intermediate temperature applications.21−23 The doped membranes show good proton conductivity at these temperatures and are thermally stable.24 However, the leaching of phosphoric acid from the membrane25 inhibits prolonged fuel cell operation. Further, phosphoric acid condenses at high temperature (via evaporation) to form polyphosphoric acid. The formation of a P−O− Pt bond at the cathode blocks the adsorption of O2, leading to a Received: August 21, 2013 Revised: October 25, 2013 Published: October 30, 2013 14449

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slow oxygen reduction. 26 While various derivatives of imidazole-based membranes have also been investigated, their conductivities are much lower than PFSA membranes.27,28 Since the past decade, application of proton-conducting ionic liquid (IL)-doped membranes29−39 as electrolytes in intermediate/high-temperature fuel cell applications has received attention. The ILs have been chosen based on properties40−42 such as high thermal and electrochemical stability, low vapor pressure, and high ionic conductivity. In particular, protic ILs, a subclass of ILs, are considered as potentially promising dopants in PEMs due to high proton conductivity.43−45 A protic IL is formed by an equimolar combination of a Brønsted acid (AH) and Brønsted base (B). The formation of the protic IL occurs via a proton transfer from the acid to base (AH + B → A− + BH+), leading to proton-donor and proton-acceptor sites, which, consequently, create a hydrogen-bonded network. A list of protic ILs and their properties and applications can be found elsewhere.46 These ILs can serve as proton carriers in the bulk phase and in doped membrane environments. These ILs also facilitate the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) at the interface of electrodes at higher temperatures (>100 °C) and under nonhumidified conditions.38,43,47,48 The potential difference between the HOR and ORR processes directly depends on the ΔpKa of the Brønsted acid−Brønsted base pair, forming the protic IL, where, Ka is the acid dissociation constant.49 In an IL-doped Nafion membrane, the cations of ILs combine with the sulfonic acid groups by partially replacing the protons causing a plasticizing effect on the polymer matrix and, consequently, reduce the swelling of the membrane.35 Proton conduction in bulk protic ILs occurs via a vehicular motion of anions and cations. However, in an IL-doped membrane, a significant contribution of structural diffusion was also observed.30,39,50 Iojoiu and co-workers51 proposed that the triethylamine (TEA)-saturated and (TEATF)-doped Nafion membranes consist of nanoaggregates comprised of protonacceptor and proton-donor sites. The authors hypothesized that these sites participate cooperatively and simultaneously in the proton-transport mechanism via short-range inter- and intracluster transfer. The authors also proposed three possible longrange proton-transport mechanisms, (a) via cationic clusters, (b) via a concerted interaction between cation and anion clusters, and (c) via direct proton-exchange between cationic and anionic clusters. Watanabe and co-workers39 proposed a different proton-transport mechanism from the anode to cathode in the PEM fuel cell. The protons generated by the HOR at the anode are accepted by free bases to form cations that are transported through the IL-doped membrane by a rapid proton-exchange mechanism between cations and free bases. Finally, cations lose protons at the cathode by the ORR to generate free bases and water molecules. The free bases can accept protons either from cations or those generated at the anode by the HOR. The limited work done so far has proposed that the role of anions of the IL is limited to enhance the hydrophobicity of the membrane.35 Though proton transfer occurs from cations to bases, the actual role of anions in proton transport is unclear and very poorly understood. The objective of this present work is to present a mechanistic detail of proton transfer using quantum chemistry calculations and to examine the role of the anion and cation. Such a study will provide a fundamental understanding on how proton transfer can occur in IL-doped PFSA membranes and can assist experimentalists in the design,

development, optimization, and application of ILs as dopants for the PEM. Our choice of PFSA membranes also stems from its potential to deliver high proton conductivity in humidified and nonhumidified environments. A Brønsted acid, trifluoromethanesulfonic acid (triflic acid (TFA), CF3SO3H), was chosen due to close chemical resemblance with the side-chain ends of PFSA membranes. TEA was chosen as a Brønsted base that reacts with TFA to generate triethylammonium cation (TEAH+) and triflate anion (TFA−), which lead to the formation of a protic IL, TEATF, applicable in intermediate/ high-temperatures PEM fuel cell devices. The computational details are presented in section 2. The mechanism of proton transport in IL-doped PFSA membranes is discussed in section 3. A summary of key results concludes this paper.

2. COMPUTATIONAL DETAILS All calculations were carried out using the Gaussian 0952 suite of programs. Four types of calculations were performed to analyze the proton-transport mechanism, (1) optimization of cations, anions, and neutral molecules followed by vibrational frequency calculations; (2) potential energy surface (PES) scans; (3) determination of transition-state structures; and (4) intrinsic reaction coordinate (IRC) calculations for the reaction pathway. All quantum chemistry calculations were performed using a hybrid density functional theory (DFT) with a Becke’s three-parameter functional (B3LYP)53−56 and a 6-311++G** basis set. An optimization of smaller species, such as TFA, TFA−, TEA, and TEAH+, was carried out using the B3LYP/6311++G** level of theory. For larger species, optimization was done in the following manner: HF/6-311G, HF/6-311++G**, and B3LYP/6-311++G**. Frequency calculations were performed on each fully optimized structure at each level of theory. Calculations were performed without imposing any symmetry constraints on the system. The energy of the optimized structure was corrected with zero-point energy (ZPE) that provides the total internal energy of the system in the gas phase. A PES scan was performed on the optimized structure by moving the hydrogen atom participating in the protontransfer mechanism. Transition-state structures were obtained by optimizing the structure representing maxima on the PES curve by employing the opt=TS keyword, which implements the Berny algorithm using GEDIIS.57 The presence of one imaginary frequency confirms the transition-state structure. IRC calculations58,59 in forward and reverse directions were performed on the transition-state structure to confirm its connectivity with reactants and products. 3. RESULTS AND DISCUSSION The chemical structure of the TEATF constituents, TFA, TFA−, TEA, and TEAH+, are shown in Figure 1. The color schemes are as follows: dark blue, light blue, dark gray, light gray, red, and yellow spheres represent N, F, C, H, O, and S atoms, respectively. An interaction of these constituents is chosen for this study to understand proton conduction in ILdoped PFSA membranes. The optimized structure of TEA and TEAH+ shows two faces, and hence, they can interact from either side. All ethyl groups in TEA or TEAH+ are oriented in the same direction with a C3 symmetry. If a Nafion membrane is doped with an IL, a partial replacement of protons of the sulfonic acid groups by cations is observed.35 Our calculations performed in this work show that proton replacement by a cation occurs through an interaction 14450

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Figure 1. Structures of TEATF constituents: (a) triflic acid (TFA); (b) triflate anion (TFA−); (c) triethylamine (TEA); and (d) triethylammonium cation (TEAH+). Dark blue, light blue, dark gray, light gray, red, and yellow spheres represent N, F, C, H, O, and S atoms, respectively.

Figure 2. Interaction of a Nafion-side-chain fragment with a TEATF IL unit. TFA− approaches toward the hydrogen atom and TEAH+ toward the oxygen atom of the sulfonic acid end group of the membrane fragment.

of an anion with the side-chain. Both TEAH+ and TFA can potentially replace protons from the side-chain fragments of PFSA membranes. A combination of their corresponding bases, TEA and TFA−, with the side-chain fragment of a Nafion membrane releases 25.27 and 30.34 kcal/mol energies, respectively, to form respective complexes. Though, the energy released in the case of TFA− is higher; the calculated energies of separation of the constituents from the complexes are −86.56 and −26.55 kcal/mol, respectively, which indicates a stronger electrostatic interaction of side-chain anions with TEAH+. The side-chain binds with a TFA− by sharing the hydrogen atom, which is closer to the triflate O atom than the side-chain O atom. This is discussed in more detail in the next subsection. In TEATF-doped PFSA membranes, both the cation and anion can access suitable sites, simultaneously, on the sulfonic acid end groups. For example, the cation approaches toward the O atom, and the anion approaches the H atom. The anion abstracts the proton from the acid and facilitates the cation to electrostatically bind to the sulfonate ends of the membranes. The structure of the resultant complex (side-chain fragment of a Nafion membrane with a TEATF) is shown in Figure 2. The atomic distances illustrate that O1, H, and O2 atoms are bonded with a medium/strong hydrogen bond, whereas the N, H, and O3 atoms are bonded by a medium strength hydrogen bond [strong H bond (1.2−1.5 Å), medium strength H bond (1.5−2.2 Å), and weak H bond (2.2−3.2 Å)].60 There also exists a weak hydrogen-bonding network between the ethyl groups of TEAH+ and oxygen atoms of sulfonates in TFA− and the side-chain fragment. In conclusion, TFA− ions facilitate binding of cations to the side-chains of PFSA membranes via abstraction of a proton from their sulfonic acid groups. The energies of formation of stable complexes from the reaction of a TEA with a TFA and a Nafion side-chain were calculated to be −24.58 and −25.27 kcal/mol, respectively, which are almost equal. Thus, the polymer matrix of a TEATFdoped Nafion membrane (or PFSA membranes) can be assumed to be a TEATF matrix consisting of some fixed TFA− ions (representing membrane anions), and the rest are free to move. The structures of anions and cations in a TEATF

matrix and the possibility of proton-exchange among them are discussed in the following sections. A. Proton-Exchange between TFA and TEA. In a TEATF matrix, along with cations and anions, TFA and TEA also exist. The transport of protons through the matrix via proton-exchange between proton donors and proton acceptors contributes to the effective proton conductivity of the IL-doped membrane. TEAH+ and TFA are proton donors, and TEA and TFA− are proton acceptors. TFA instantly donates its proton to TEA. Figure 3 shows the two most probable combinations of TFA and TEA, according to the interacting face of a TEA, leading to stable TEATF complexes. In complex (a), ethyl

Figure 3. Formation of a TEATF unit from the reaction of a TEA and a TFA. Complexes formed by the reactions are in (a) “extended” and (b) “collapsed” forms obtained according to the approaching face of a TEA molecule toward the hydrogen atom of a TFA. 14451

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TFA− is shown in Figure 5a. This is the most stable structure of the TFA···TFA− complex among all possible isomers (different initial guess structures were used). The complex shows the orientation of −CF3 groups on the same side, resembling the proposed “micelle” structure of TFA− by Iojoiu and coworkers.30 The atomic distances in the structure show that the H atom prefers to be shared by TFA and TFA− as compared with the Nafion−TFA− complex. Figure 5b is the transitionstate structure. The proton-exchange between TFA and TFA− occurs with a very small barrier of ∼0.06 kcal/mol (the ZPE is not included). The ZPE of the transition-state structure is lower than that of the reactant, which again concludes that the H atom is equally shared by TFA and TFA−, due to the superacidic nature of a TFA. C. Proton-Exchange between TEAH+ and TEA. The basicity of amine is known to increase with increasing electronreleasing substituent on the N atom. However, the steric hindrance of bulky groups inhibits the N atom from interacting with other molecules, such as water or another amine, and, thus, decreases the aqueous basicity. A TEA can interact with a TEAH+ through a weak hydrogen bond producing two stable TEAH+···TEA complexes (a) and (b), as shown in Figure 6.

groups exist in an “extended” form, whereas in complex (b), they remain in a “collapsed” form, releasing 24.58 and 18.87 kcal/mol of energy, respectively. Thus, the configuration of TEATF with extended ethyl groups is more stable. The atomic distances between connected N, H, and O atoms in both TEATF complexes are similar and show a medium strength hydrogen bonding, which indicates an electrostatic interaction between TEAH+ and TFA−. An examination of other O−H distances in the complexes (shown by dotted lines in the figure) indicates that H atoms of ethyl groups also participate in a weak H-bonding network with O atoms of TFA−. B. Proton-Exchange between TFA and TFA−. Figure 4 shows the optimized structure of the side-chain fragment−

Figure 4. Interaction of a Nafion−side-chain fragment with a TFA−.

TFA− complex. The structure shows that the hydrogen atom is nearer to triflate O atom (O−H = 1.07 Å) and is also strongly hydrogen-bonded with a side-chain O atom (O−H = 1.39 Å) that is of covalent character. Clearly, the proton dissociates from the side-chain fragment and binds with an O atom of TFA−. A PES scan and subsequent optimization of the sidechain fragment−TFA− complex, performed by moving the hydrogen atom from TFA− to the side-chain fragment, also confirms it by showing increasing energy of the complex. In the TEATF matrix, proton-exchange can also occur between a TFA and a TFA−. The optimized structure of the complex TFA···

Figure 6. Interaction of a TEAH+ with a TEA leading to (a) a staggered structure and (b) an eclipsed structure of the TEAH+···TEA complex.

Structure (a) is the “staggered” form of the complex, and structure (b) is the “eclipsed” form. When all ethyl groups are oriented in the same direction, the staggered form is the most stable, and if they are in opposite direction, the eclipsed

Figure 5. A schematic of the PES of proton-exchange in the TFA···TFA− complex. 14452

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Figure 7. Effect of a TFA− interaction with the TEAH+···TEA complex on proton transfer.

distance slightly from 1.06 to 1.04 Å and, thus, weakens the proton-exchange ability of TEAH+ by reducing its acidity. The reduction in acidity of TEAH+ with the interaction of a TFA− implies increased basicity of its conjugate base (TEA). To examine this and its effect, TFA− was placed on the right side of the TEAH+···TEA complex, and the resulting system was optimized. The final complex is shown in Figure 7b and has the same configuration as in structure (a). This shows that the transfer of a proton occurs from TEAH+ to TEA during optimization. Figure 8 shows the optimization process of TEAH+···TEA···TFA−. The structures shown in the figure are the configurations of the complex at the points denoted by blue circles, and the moving hydrogen atom is denoted by a red circle. A steep decrease in system energy occurs due to TFA− interaction with the TEAH+···TEA complex. Structure (b) shows the transfer of a proton from TEAH+ to TEA, leading to

structure is the most stable. The energies of formation of both structures are almost the same, −8.35 and −8.25 kcal/mol, respectively. In both complexes, the N−N, N−H (bonded), and N−H (nonbonded) atomic distances are 3.11, 1.06, and 2.05 Å, respectively. This shows a medium strength/weak N− H···N hydrogen-bonding network in these complexes due to steric hindrance of ethyl groups. Proton transfer from TEAH+ to TEA requires sufficient proximity between the N and H atoms, and in this case, quantum chemistry calculations show that proton transfer is not possible. We also examined protonexchange between primary, secondary, and other tertiary amines with their corresponding cations. Except for tertiary amines, a barrierless proton-exchange occurs between primary/ secondary amines with their corresponding cations because of their higher basicity and lower steric hindrance of the electronreleasing substituent. D. Interaction of One TFA− with a TEAH+···TEA Complex. Previous29,38 work has proposed proton-exchange between cations and free bases as a mode of proton conduction in IL-doped membranes.30,39 Our calculations show that the proton transfer only between the TEAH+ and TEA could not be achieved. To examine this further, interaction of a TEAH+··· TEA complex with other IL constituents was considered. Of all possibilities, only an interaction of TFA− with the TEAH+··· TEA complex leads to proton transfer between TEAH+ and TEA. A TFA− can access the TEAH+···TEA complex from the left or right. If approached from the left, TFA− binds with ethyl groups via a hydrogen-bonding network (as shown by dotted lines in Figure 7a along with the atomic distances). The hydrogen atom likely to participate in the proton-exchange mechanism is shown by a red circle. The N1−N2 distance in the TFA−···TEAH+···TEA complex is larger compared to that in the TEAH+···TEA complex. This is due to electrostatic interaction between the TEAH+ and TFA−. A TFA− interaction with the TEAH+···TEA complex also decreases the N−H bond

Figure 8. Mechanism of proton transfer in a TEAH+···TEA···TFA− complex. 14453

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Figure 9. A schematic diagram of the potential energy to illustrate proton transfer in the TFA−···TEAH+···TEA···TFA− complex.

change of orientation of TEAH+ in the complex with an activation energy barrier for proton transfer, the proton conductivity will be relatively low.50 However, an increase in TEATF concentration can lead to an increase in free-moving and proton-donating TEAH+, which can enhance proton conductivity.

structure (c). An examination of the N1−N2 and N1−H distances shows that TEAH+ moves closer to TEA in the complex, releases a proton, and returns back. During the fuel cell operation, amines are continuously generated at the cathode by ORR. If these amines are hydrogen-bonded with sulfonate end groups of side-chains or triflate ions, they can instantly take protons from freely moving cations, reproducing free amines. The resultant free amines can accept protons either from cations or from those generated at the anode, as proposed by Watanabe and co-workers.39 E. Interaction of Two TFA− with a TEAH+···TEA Complex. It is not always possible to find a freely moving cation (TEAH+) in the IL-doped Nafion−polymer matrix to donate a proton to a TEA. These cations can electrostatically interact with anions. Proton-exchange between TEAH+ and TEA can also occur via formation of a TFA−···TEAH+···TEA··· TFA− complex in the polymer matrix. Figure 9 shows a schematic of PES of this proton-exchange mechanism. Structures representing (a), (b), and (c) on the curve are initial, transition-state, and final structures, respectively. Structures at (a) and (b) are the same in the configuration. The moving hydrogen atom in the TFA−···TEAH+···TEA··· TFA− complex is denoted by a red circle. The N−N distance in the initial structure is 3.04 Å, which is slightly lower than that in the TEAH+···TEA complex (3.11 Å). This shows that the participation of two TFA− decreases the distance between TEAH+ and TEA to facilitate proton transfer. In the transitionstate structure (b), the N−N distance is 2.68 Å, and the H atom is at the midpoint of the two nitrogen atoms. The activation energy barrier for the proton transfer is 2.62 kcal/mol. In conclusion, proton-exchange in TEAH+···TEA can occur only if either TEA or both TEA and TEAH+ interact with TFA−. If a Nafion membrane is doped with only TEA, TEA can abstract protons from sulfonic acid end groups, forming cations (TEAH+), which will remain electrostatically attached with side-chain fragments as a side-chain···TEAH+ complex. The ORR at the cathode of the fuel cell leads to the formation of side-chain···TEA. This complex can only abstract a proton from the nearest side-chain···TEAH+ complex. Because it requires a

4. CONCLUSIONS Proton transport in TEATF-doped PFSA membranes were characterized using quantum chemistry calculations. A TEATF unit, formed by a proton transfer from TFA to TEA, is the combination of TEAH+ and TFA−, which are bound by electrostatic interactions. Results show that TFA− and TEAH+ can potentially bind with the sulfonic acid end groups of sidechain fragments. A TFA− approaches toward the acidic hydrogen atom and takes it to produce a TFA, and a TEAH+ binds with an electrostatic interaction with the resultant sulfonate anion end in the fragment. There are only three possible ways of proton transfer, TFA to TEA, TFA to TFA−, and TEAH+ to TEA. Among them, the TEAH+···TEA complex does not show proton transfer. An interaction of the TEAH+··· TEA complex with a TFA− from the right side shows a transfer of a proton from TEAH+ to TEA, producing TEA···TEAH+··· TFA−, without any barrier; that is, a proton slides rapidly from TEAH+ to TEA. If the TEAH+···TEA complex interacts with TFA− from both sides to form a TFA−···TEAH+···TEA··· TFA− complex, proton transfer from TEAH+ to TEA still occurs with a small activation energy barrier. This shows that anions play a critical role in proton transfer in TEATF-doped PFSA membranes. The conclusions derived from this study can motivate experimental investigations to design and develop various ILs and membrane materials suitable for intermediate/ high-temperature fuel cell applications. An application of ab initio molecular dynamics simulation methods to characterize dynamical properties of IL-doped membranes can also be the focus of future activities. 14454

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +91-20-2590 8085. Fax: +91-20-2586-5315. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work used the computing resources provided by the Indian Institute of Science Education and Research, Pune (IISER Pune). M.K. thanks IISER Pune for a postdoctoral fellowship. The authors thank DST (Grant No: SB/S1/PC015/2013) and DST Nanomission (Grant No: SR/NM/NS42/2009 and SR/NM/NS-15/2011) for generous financial support towards this work.

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